We are presently engaged in a study to evaluate the efficacy of the drug cysteamine (2-aminoethanthiol) in children with nephropathic cystinosis. This drug is able to deplete cultured cystinotic fibroblasts of their abnormal stores of free-cystine. When taken orally, it also depletes these children's white blood cells of over 90% of their abnormally high free-cystine content. We presently have 53 patients formally enrolled in the study. The dose of cysteamine required to deplete the patient's white blood cells by 90% is individually determined for each patient. Each patient's renal function is evaluated every four months and the data is compared to a control group of cystinotic patients from a previous study. Preliminary results are encouraging. However, we feel it is necessary to completely establish the value of this drug and to be certain that toxicity does not develop when it is administered for a prolonged period of time.